a) Field of the Invention
The present invention relates to heat exchangers which operate in counter-current and which are used in plants for the batch treatment of fluids.
b) Description of Prior Art
These plants cause particular problems, for the following reasons.
In continuous operation, a heat exchanger operating in counter-current has a temperature curve which is generally linear between its cold end and its hot end.
Since this curve is bound to the temperature of the fluids which pass through the heat exchanger and which exchange heat with one another, any sudden pause of the circulation of these fluids produce a rapid standardisation, by conduction, of the temperatures of the exchanger towards a temperature which is substantially the average of the temperatures from the hot end to the cold end.
The exchanger therefore undergoes rapid variations of temperature at its ends, and a major risk of deformation or breaking down appears when it is restarted, because of the thermic shocks produced by the fluids treated.
For example, in the case of the main heat exchanger of a plant for the distillation of air and the production of nitrogen of the type HPN (High Purity Nitrogen), the air treated at 8 bars enters at +20.degree. C. and is cooled to about -169.degree. C. in counter-current to the products which exit: nitrogen, reheated from -173.degree. C. to +15.degree. C. and the residual gas, reheated from -180.degree. C. to +15.degree.C. In permanent operation, the exchanger has a temperature which varies linearly from about -175.degree. C. at the cold end up to +17.degree. C. at the hot end. If the circulation of fluids is suddenly stopped, the temperature of the exchanger rapidly reaches an equilibrium at about -80.degree.C.